The present invention relates to a vehicle running control apparatus, and more particularly to a vehicle running control apparatus and method in which a vehicle to be controlled is caused to run with a transmission mechanism, a throttle mechanism and a brake mechanism controlled so that a distance between the vehicle to be controlled and an object such as a preceding vehicle existing in front of the vehicle to be controlled is matched with a desired distance.
In recent years, there have been developed a vehicle running control apparatus provided with a vehicle speed control function (or cruise control function) of performing the running of a vehicle to be controlled with a vehicle speed kept constant as well as a headway distance control function (or adaptive cruise control function) of performing the running of the vehicle with a safe headway distance kept between the vehicle and a preceding vehicle by use of a radar system which is mounted on the vehicle to be controlled and detects a headway distance to the preceding vehicle. As has been disclosed by, for example, JP-A-7-76237, a follow-up running for a preceding vehicle is performed by controlling a vehicle speed so that a headway distance detected by a radar sensor keeps a safe headway distance corresponding to the vehicle speed. In the case where there is no preceding vehicle within the safe headway distance, an acceleration running is performed up to a preset desired speed (or target speed command) and a keep-speed running is thereafter performed in accordance with the desired speed. Also, there is an apparatus in which when a host vehicle to be controlled is running on a curved road and a preceding vehicle having been able to be measured by a radar system moves out of a measurable range so that the measurement becomes impossible, the speed of the preceding vehicle is estimated and the speed of the host vehicle is controlled by use of the estimated speed.
JP-A-9-323628 corresponding to the U.S. patent application Ser. No. 08/871,887 filed on Jun. 9, 1997 has disclosed an apparatus in which a vehicle speed control is performed while maintaining a headway distance to a preceding vehicle at a required value. JP-A-10-76867 corresponding to the U.S. patent application Ser. No. 08/923,414 filed on Sep. 4, 1997 has disclosed a running control system in which a running control map corresponding to the running condition of a vehicle provided with an automatic cruising device is prepared so that an area of the running control map is selected in accordance with a relative speed and headway distance to a preceding vehicle.
The radar systems used in the above-mentioned apparatuses include three typical systems as enumerated in the following. The first system is a laser radar. The principle of the laser radar will now be described in brief. A short pulse-like laser beam is periodically transmitted (or emitted) from a laser diode, and a laser beam reflected by a reflecting plate such as a reflector mounted at the rear of a vehicle is received by a photodiode. A distance to the reflecting plate is measured from the product of a time from the transmission to the reception and the light velocity. Since an object having the reflecting plate mounted thereon is detected, it is easy to detect, for example, a motor vehicle or an auto-bicycle. On the other hand, under a condition in which the visibility with the naked eyes is unavailable owing to, for example, rain, fog or the like, a detectable distance becomes remarkably short as compared with that at the time of fine weather. This is because there is used a laser beam which is easy to handle and has a wavelength close to visible light. Also, since the principle of measurement is based on the measurement of a time from transmission to reception, what can directly be detected is only a distance.
The second system is a frequency modulated continuous wave (FMCW) radar. An electromagnetic wave or a radio wave signal having a continuously changing frequency are transmitted and an electromagnetic wave signal reflected from an object (or receive wave) are received. The transmit wave and the receive wave are mixed to obtain a signal which has a frequency corresponding to a distance to the object reflecting the transmit wave and a signal which has a frequency corresponding to a relative speed. Thereby, it is possible to detect the distance and the relative speed directly. For example, provided that the center frequency of the transmit wave is 60 GHz, a modulation band is 75 MHz and a frequency for causing a frequency change is 750 Hz, a distance detection range of the radar is 150 m and the resolution of the relative speed is 3.75 m/s. The wavelength is long as compared with that in the laser radar and an object existing in the front can be detected even under a condition in which the visibility with the naked eyes is unavailable.
The third system is a two-frequency continuous wave (or two-frequency CW) radar. Two different frequencies f.sub.1 and f.sub.2 are transmitted in a time-shared switching manner so that a relative speed and a distance to an object are detected from a Doppler frequency or a frequency change which is included in waves reflected from the object (or receive waves). For example, provided that a first frequency of the transmit waves is 60 GHz and a second frequency thereof is 60 GHz+250 kHz, a distance detection range of the radar is 150 m. Similarly to the FMCW system, the wavelength is long as compared with that in the laser radar and an object existing in the front can be detected even under a condition in which the visibility with the naked eyes is unavailable. Also, there is a feature that the resolution and accuracy of the relative speed are better or equal to or smaller than 0.3 m/s. However, since the frequency change caused by the Doppler effect is used as the principle of measurement, it is not possible to detect an object when the relative speed is zero.
JP-A-8-166444 has disclosed a distance measurement equipment based on the two-frequency CW system.